1. Field of the Invention
This invention relates to a method for driving a nonvolatile semiconductor memory device based on transistor type memory cells having an insulating film charge storage layer.
2. Background Art
With the recent progress of downscaling, a NAND flash memory having a floating gate involves the problem of noticeable interference between adjacent memory cells due to the thickness of the floating gate itself as well as the difficulty in burying an insulating film between the cells.
To avoid such problems specific to floating gate flash memories, a so-called MONOS flash memory is proposed (e.g., JP-A 2005-011490(Kokai) and US Patent Application Publication No. 2005/0006698). “MONOS” is an abbreviation for the “metal-oxide (oxide film)-nitride (silicon nitride film)-oxide (oxide film)-silicon” structure. It is characterized in that the data memory state is changed by causing discrete traps existing in the charge storage layer such as a silicon nitride film to capture charges. Like the oxide film, the charge storage layer is also an insulating film and allows thin film formation. Hence the above problems occurring in floating gate flash memories can be resolved. The number of discrete traps and the depth of the traps in the charge storage layer affect the capturing efficiency and charge retention capability at the time of charge injection. Hence, besides the silicon nitride film, application of a metal oxide film and the process condition therefor are under development.
In a MONOS flash memory, charges are captured by discrete traps. Hence the data memory state depends on the position of the discrete traps and the position of the captured charges. Likewise, the charge capturing efficiency depends on the charge distribution in the charge storage layer before capturing the charges. Furthermore, in the charge retention state, the captured charges may be redistributed in the charge storage layer to change the data memory state.
However, the operation for changing the data memory state and reading the data used in the MONOS flash memory is similar to the operation in conventional floating gate flash memories. There has been no investigation on the method specific to the MONOS flash memory for electrically controlling the charge distribution in the charge storage layer. More specifically, there has been no investigation on how the charge capturing efficiency and the data memory state are affected by controlling the charge distribution in the charge storage layer, and the method for electrically controlling the charge distribution has yet to be established.